you will never see a complete overhaul. the basic interface design was done once they opened up the APIs to developers
they can't change the icon size without breaking existing icons. they can't change the transparency without breaking programs. they can't change the resolution without breaking programs.
most of all, you train users to get used to icons in certain places, certain colors, etc. they can't change this without annoying > 95% of their users.
the same old basic UI will be around for years. Apple spent years designing it, they're not just going to throw it out.
It will happen - sooner or later. They will eventually have to move forward or they will eventually end up with a very dated device. Whether they bring out a completely new device with a fresh UI or not. It's just something that will have to be done.